1. Field of the Invention
Apparatuses and a method consistent with the present invention relate to a magnetic tape cartridge including a magnetic tape within on which servo signals are written, a servo writer for writing servo signals on a magnetic tape, a magnetic tape drive for recording/reproducing data on/from a magnetic tape, and a method for reading servo signals.
2. Description of the Related Art
In recent years, a high density recording design of magnetic tapes has advanced, and some of magnetic tapes for backup media of computers have a recording capacity of several hundreds of gigabytes. Magnetic tapes therefore have several hundreds of data tracks along the width thereof. Such a high density recording design involves excessive narrowing of the data tracks and of the intervals between the adjacent data tracks in a magnetic tape. Thus, in order to allow recording/reproducing devices of a magnetic head to trace such narrow data tracks, servo signals are written on a magnetic tape in advance, and the servo signals are then read by a magnetic head, while the position of the magnetic head relative to the magnetic tape (the position along the width of the magnetic tape) is servo-controlled (refer to Japanese Unexamined Patent Application No. 8-30942 (Paragraph No. 0016 and FIG. 1)).
The servo signals are written on corresponding non-magnetized servo bands on a magnetic tape by magnetizing the servo bands in one direction by use of a head of a servo writer, to which a record current is supplied. Specifically, in a conventional technique as shown in FIG. 11A, a record pulse current PC having positive pulses PP and zero pulses ZC is fed to a magnetic head as a record current, and servo signals SS are thereby recorded on non-magnetized servo bands on a magnetic tape. In use of this record pulse current PC, as shown in FIG. 11B, when the zero pulses ZC of the record pulse current PC is fed to the magnetic head, the magnetic head does not magnetize the servo patterns SP. Meanwhile, when the positive pulses PP are fed thereto, parts (servo patterns SP) of the servo bands SB are magnetized in one direction, due to leakage flux from servo gaps of the magnetic head. As a result, the servo signals SS are written on the corresponding servo bands. Each space between the servo bands SB adjacent to each other serves as a data band DB on which a data signal is to be written.
Each servo pattern SP is composed of bursts Ba and Bb. The burst Ba is a magnetized portion of two stripes, which are both inclined at a positive angle with respect to a running direction of the magnetic tape MT. The burst Bb follows the burst Ba, and is a magnetized portion of two stripes, which are both inclined at a negative angle. These servo patterns SP are repeatedly formed at predetermined intervals, thereby constituting the servo signal SS. In these servo patterns, some variations can be conceived as appropriate. For example, the bursts Ba and Bb maybe formed of five positive inclined stripes and five negative inclined stripes, respectively. Alternatively, the servo signal SS may be constituted of two types of servo patterns alternately repeated, one of which is formed of five positive inclined stripes and five negative inclined stripes, and the other of which is formed of four positive inclined stripes and four negative inclined stripes. Note that the servo patterns SP are enlarged relative to the magnetic tape MT in FIG. 11B for clarity.
A magnetic tape drive uses a servo signal sensing element (MR element) to sense a variation in magnetic force generated on the servo signal SS, based on a variation in an electric resistance of the servo signal sensing element itself, and outputs the sensed variation in a differential waveform (voltage value) Accordingly, as the variation in the electric resistance of the MR element is increased, the peak value of the voltage signal being read from the servo signal is higher, that is, the SN ratio of the voltage signal is enhanced. Specifically, if the variation in the magnetic force generated on the servo signal SS is increased, or if the servo signal sensing element (MR element) has a large sensing area, then the voltage signal RSL being read from the servo signal SS has a high peak value, as shown in FIG. 11C.
In the future, it is expected that magnetic tapes will develop to the extent that they will have a recording capacity of several tens of terabytes. As such high density recording designs proceed, the number of data tracks formed on a magnetic tape is increased, the width of data tracks and interval between data tracks adjacent to each other are further narrowed, and a magnetic tape itself is thinned. This involves weakening of a magnetic force sensed upon reading of servo signals, and a reduction in a variation thereof. This causes lowering of the peak value of the voltage signal being read from the servo signal SS, as show in FIG. 11D, and the SN ratio of the voltage signal is thus deteriorated. Consequently, a magnetic tape drive fails to read the servo signal SS correctly, and to exactly control the position of the magnetic head.
To overcome this disadvantage, a technique disclosed in JP 2003-110396 (non-published) has been conceived by this inventor. In this technique, as shown in FIG. 12A, a DC erase head (not shown) magnetizes the servo bands SB in one direction (forward direction) along the long side of the magnetic tape MT (DC magnetization) and, then records the servo signals SS on the servo bands SB by magnetizing the parts of the servo bands SB in the opposite direction (reverse direction). In this figure, the magnetized directions are denoted by small arrows. The level of the voltage signal, which is read from the servo signal SS by the servo signal sensing element, depends on the variation in the magnetic force at a border where the orientation of the magnetization is changed. In this case, the orientation of the magnetization is greatly changed from the forward to reverse directions at a border between the regions forwardly magnetized and reversely magnetized. Similarly, the orientation of the magnetization is also greatly changed from the forward to reverse directions at a border between the regions reversely magnetized and forwardly magnetized. Consequently, thanks to this large variation of the magnetic force, a high level of the voltage signal can be obtained, as shown in FIG. 12B. In other words, the SN ratio of the voltage signal can be improved.
However, the level of the voltage signal may be much higher or lower than that of a conventional thick magnetic tape that is not subjected to the DC magnetization. Concretely, if the level of a voltage signal obtained from the conventional magnetic tape is assumed to be 100%, then that obtained from the current magnetic tape may be 200% or 70%. This is quite difference from 100%. Upon occurrence of such a great difference, a conventional magnetic tape drive is hard to record/reproduce data on/from the magnetic tape MT being DC-magnetized.
A conventional magnetic tape drive has an AGC (auto gain controller) that fine-adjusts the variation in the voltage signal to an allowable range, and this AGC is designed to utilize the conventional level (100%) as a reference. If the adjustable range of an AGC is within ±50%, then the AGC can adjust the level of the voltage signal to be 100% even when the level is varied from 50 to 150%. A magnetic tape drive can therefore read the servo signals.
When a magnetic tape cartridge having DC magnetized servo bands SB is set at such a conventional magnetic tape drive, the drive recognizes that the servo bands SB are not DC magnetized. It is assumed that the average level of voltage signal of the DC magnetized magnetic tape cartridge is 70% relative to the conventional level. If this voltage level is varied over a range of ±50%, that is, 35% to 105%, then the lowest level is below 50% and, thus falls outside the adjustable range (50% to 150%) of an AGC. As a result, the magnetic tape drive may fail to read the servo signals. Next, it is assumed that the average level of the voltage signal is 200%. Even if this servo signals is normally read, the output level falls outside the adjustable range of an AGC. As a result, a magnetic tape drive may fail to read the servo signals, as well.
To solve the above problem, is requested, a magnetic tape drive which recognizes an output level of servo signals, and which adjusts the level to an allowable level. In the future, two types of magnetic tape cartridges, that is, a magnetic tape cartridge having DC-magnetized servo bands and a magnetic tape cartridge having non-DC-magnetized servo bands will be used in combination. In this case, the above magnetic tape drive is especially required.
An object of the present invention is to provide a magnetic tape cartridge and a servo writer which both allow a magnetic tape drive to recognize a level of a voltage signal being read from a servo signal.
Another object of the present invention is to provide a magnetic tape drive and a method for reading a servo signal, which both makes it possible to recognize a level of a voltage signal being read from a servo signal, and to adjust this level to an allowable level.